1. Field of the Invention
The present invention relates to a semiconductor device having an image sensor function and to a method of manufacturing the same. More specifically, the present invention relates to a semiconductor device in which a photoelectric conversion element and a transistor are formed on an insulating surface and to a method of manufacturing the same.
Note that, in this specification, the term semiconductor device generally refers to devices hat utilize semiconductor characteristics to function, and includes as a category electrooptical devices, semiconductor circuits, and electronic equipment.
2. Description of the Related Art
In recent years, various kinds of sensors have been developed and put into practical use with accompanying technology advancement thereof. One type of those sensors are semiconductor devices having an image sensor function, which are used to capture text and image information on paper or the like into personal computers.
Examples of this type of semiconductor devices include digital still cameras, scanners, and copying machines. Digital still cameras are used as replacements for conventional silver film cameras, and have area sensors in which pixels are arranged two-dimensionally. Scanners and copying machines are used as means for reading text and image information on paper, and have line sensors in which pixels are arranged one-dimensionally.
A semiconductor device having an image sensor function is provided with a pixel portion that has a plurality of pixels. Each of the pixels has a photoelectric conversion element and one or a plurality of transistors selected from a transistor that functions as a switching element of the photoelectric conversion element, a transistor for amplifying a signal of the photoelectric conversion element, and a transistor for erasing a signal of the photoelectric conversion element.
A photoelectric conversion element often used is a PIN photodiode. Also used are a PN photodiode, an avalanche diode, an npn embedded diode, a Schottky diode, and a phototransistor. An x-ray photoconductor and an infrared sensor are also photoelectric conversion elements.
Semiconductor devices having an image sensor function are roughly divided into CCD type and CMOS type. CMOS type semiconductor devices are further classified into passive semiconductor devices to which amplifying transistors are not mounted and active semiconductor devices to which amplifying transistors are mounted. An amplifying transistor has a function of amplifying an image signal of a subject read by a photoelectric conversion element.
An active semiconductor device has, in addition to an amplifying transistor as above, a sensor selecting transistor and the like. Accordingly, the number of elements in one pixel in an active semiconductor device is larger than in a passive semiconductor device.
When manufacturing the semiconductor devices described above, a transistor is formed first on an insulating surface and then a photoelectric conversion element is formed. The photoelectric conversion element is often a PIN junction element obtained by layering three thin films. Accordingly, manufacture of a semiconductor device having a photoelectric conversion element and a transistor requires at least three masks for forming a p type semiconductor layer, a photoelectric conversion layer, and an n type semiconductor layer, in addition to the number of masks for forming the transistor.
In manufacture of a semiconductor device that has a transistor and a photoelectric conversion element on an insulating surface, the manufacturing process is complicated and has a large number of steps. An increase in number of steps leads to increased manufacturing cost as well as reduction in manufacture yield.
When three additional masks are required for forming a p type semiconductor layer, a photoelectric conversion layer, and an n type semiconductor layer, in addition to the number of the masks for forming a transistor on an insulating surface, the manufacturing process of the semiconductor device becomes laborious and the yield could be greatly reduced since lowering in yield in each step is multiplied in the final yield. Furthermore, the additional masks prolong manufacture time and therefore increase manufacturing cost.
Manufacture of the semiconductor device employs the photolithography technique. The photolithography technique uses a photo mask to form on a substrate a photoresist pattern that serves as a mask in an etching step. The introduction of one photo mask involves addition of various steps such as formation of a coat, etching of the coat, resist peeling, washing, and drying in the resist application step, pre-bake step, exposure step, development step, and post-bake step, and in the preceding and following steps thereof. The additional steps make the manufacture of the semiconductor device even more laborious. Reducing steps in number can be an effective measure in improving the productivity and yield. However, manufacturing cost can be reduced only to a limited degree unless the number of photo masks is reduced.